Difluorenylsilanes, -germanes, and -stannanes Exhibiting an Unprecedented Parallel Arrangement of the Fluorene Units

New, yet unprecedented molecular structures with parallel fluorene units were found by
X-ray diffraction in (9,9‘-disubstituted difluorenyl)(dimethyl) derivatives of group 14 elements
Me2E(CR‘R2)(CR‘ ‘R2) (E = Si, R‘ = Me, R‘ ‘ = SiMe3, 6a; E = Ge, R‘ = Me, R‘ ‘ = SiMe3, 6b;
and E = Sn, R‘ = R‘ ‘ = SiMe3, 5c; CR2 = fluorene units). This is in sharp contrast with the
structure of monosubstituted Me2Ge(CHR2)(CR2-SiMe3) (3b) and disubstituted Me2E(CMeR2)2
(E = Ge, 4b; E = Sn, 4c) derivatives where the two fluorene units are almost perpendicular.
Intramolecular C−H···π interactions are revealed by solution 1H NMR spectroscopy and
supported by the AM1 molecular orbital calculations. Intermolecular (aryl, alkyl)C−H···π
interactions leading to supramolecular associations are shown by the crystal structures of
3b, 4b, 4c, 5c, and 6a. In 4b and 5c rather unusual CH···HC distances resembling the
“dihydrogen” bonds are noticed. Synthesis, NMR spectra, and gas phase conformations
predicted on the basis of AM1 semiempirical molecular orbital calculations are discussed
for Me2E(CR‘R2)(CR‘ ‘R2) (E = Si, R‘ = R‘ ‘ = SiMe3, 5a; E = Ge, R‘ = R‘ ‘ = SiMe3, 5b; E =
Sn, R‘ = H, R‘ ‘ = Me, 2c; R‘ = H, R‘ ‘ = SiMe3, 3c; R‘ = Me, R‘ ‘ = SiMe3, 6c; CR2 = fluorene
units).